I agree AVG is good. I also use Microsoft Security Essentials. Use BOTH and you will be happier.
Get SpyBot and CCleaner, too.
I haven't use paid virus protection for years. About 50-75% of the USB's my students bring in have viruses. That is the largest source of potential infection for me.
Jim Droege
Karachi, Pakstan

One of the ways companies manage to give their software away for "free" is by promoting some other software during the install process. In other words while installing the software you want a message during the install process appears that has a tick box somewhere (sometimes these are quite prominent and sometimes they are not very prominent at all) and next to the tick box there will be a question or a statement that allows the installation of some other piece of software as well as the software you were installing in the first place.
One must take great care here ! The extra software may change your browser or the browser settings. It may install toolbars within your current browser or the new browser. The point is, the third party software may not be so trustworthy. They may send you to other websites to which they receive revenue for but these could be dodgy sites that have malware on them. Or the third party software may have malware characteristics themselves, such as adware or worse actual viruses and trojans.
The point is when installing freeware it can end up there's even more malware than you started with if you are not careful.
The solution to this is to be extremely viligent during the install procedure and be sure you read what it says next to any tick boxes and also the title of the page or window you are viewing. If there is any reference to software other than the one you intend installing be very careful. If you are not sure get advice from someone more technical.